1. Field of the Invention
This invention relates to an apparatus for pulling silicon single crystal, and more particularly to an improvement of a temperature keeping cylindrical body and/or a crucible for use in the silicon single crystal pulling apparatus.
2. Description of Related Art
The conventional apparatus for pulling silicon single crystal has a structure based on so-called Czochralski method, in which silicon single crystal is pulled from silicon melt in a crucible in the presence of an atmosphere gas, and is known as a single crystal production apparatus as disclosed, for example, in JP-B-57-15079. In this apparatus, as shown in FIG. 1, a rotating shaft 2 is inserted into a furnace vessel 1 from a bottom thereof and a crucible 4 is placed on a support base 3 disposed on an end face of the rotating shaft 2, while a heating member 5 and a temperature keeping cylindrical body 6 are arranged around the crucible 4 to provide a silicon melt 7 in the crucible 4. On the other hand, a rotating shaft 9 is arranged above the furnace vessel 1 so as to slide in the furnace vessel in up and down directions. A seed crystal 8 of silicon is attached to a free end of the rotating shaft 9 and moved upward from a state of contacting with the melt 7 in the crucible 4 by the upward sliding of the rotating shaft 9 during the rotation of the shaft to provide a single crystal 10 of silicon on a bottom face of the seed crystal 8 at a state of being integrally united with the seed crystal 8.
In the growth of the single crystal, it is required to eliminate an unnecessary reaction product gas from the inside of the furnace vessel for preventing the reaction of the gas with the single crystal 10 and a liquid face of the melt 7. Therefore, an inert gas 11 such as argon or the like is used as an atmosphere gas and is fed from an inert gas source (not shown) through a pipe system 12, 13 and 14 to an upper portion of the furnace vessel 1 and then discharged together with a reaction product gas 15 from the bottom of the furnace vessel 1 through a pipe 16 to the outside of the vessel. Moreover, a heat insulating material (not shown) is arranged between the temperature keeping cylindrical body 6 and the furnace vessel 1.
In this type of the silicon single crystal pulling apparatus, the temperature keeping cylindrical body, crucible and the like are required to have an excellent heat resistance and hardly react with various metals, so that they are made from a carbonaceous material such as graphite or carbon-bonded carbon fiber composite (hereinafter referred to as C/C composite simply). Particularly, a portion of the crucible directly contacting with the silicon melt is made from quartz for preventing the inclusion of an impurity such as metal or the like into the silicon melt, while the remaining portion of the crucible is made from graphite or C/C composite having an excellent heat resistance as a crucible protection part for preventing the softening deformation of the quartz crucible part because the crucible is heated above 1400.degree. C. for the formation of silicon melt.
In the conventional apparatus, the silicon melt is formed by heating and melting a silicon material in the crucible by means of the heating member, so that SiO.sub.2 constituting the quartz crucible part and Si melted in the crucible are vaporized and dispersed in the apparatus. Further, the temperature keeping cylindrical body and the crucible, particularly upper regions thereof are exposed to a higher temperature above 1400.degree. C. required for the formation of the silicon melt and SiO gas produced from the quartz crucible part and silicon melt, so that the temperature of these upper regions becomes higher than the other remaining regions.
Therefore, it is considered that the following chemical reaction is caused in the upper regions of the temperature keeping cylindrical body and the crucible by carbon existing in these regions and SiO gas dispersed from the quartz crucible part and silicon melt: EQU SiO+2C.fwdarw.SiC+CO
As a result of such a chemical reaction, the carbonaceous material constituting the temperature keeping cylindrical body or the crucible protection part is rendered into a silicon carbide on the surface of the upper region. The resulting silicon carbide is different from the carbonaceous material in the properties, so that when the apparatus is cooled after the stop of operation, the presence of the silicon carbide creates cracks in the cylindrical body or the crucible protection part to bring about the deterioration of the durability. If the cracks are created in these parts, pieces of the silicon carbide and the carbonaceous material are peeled off from these parts to contaminate the inside of the apparatus. These pieces result in the crystal defect of the resulting silicon single crystal to be pulled with a higher purity, so that the formation of silicon carbide should be controlled surely.
Moreover, when Si vapor dispersed in the apparatus contacts with the lower region of the temperature keeping cylindrical body of a relatively lower temperature, it is cooled and liquefied to adhere (deposit) onto the surface of the lower region in form of droplets, which penetrates into a gap between the temperature keeping cylindrical body and the support base. As a result, the liquefied Si serves as an adhesive between the cylindrical body and the support base and hence the exchange of the temperature keeping cylindrical body is impossible.